There are numerous methods for measurement of body composition. A recent review article lists some twenty-three distinct methods being used or investigated for the estimation of body fat percentage. See Brodie, D. A.: Techniques of measurement of body fat composition, Sports Medicine 1988; 5:11-40 (Part I) and 5:74-98 (Part II); Forbes, G. B. (1987) Human Body Composition, New York, Springer-Verlag. This large list of methods demonstrates the importance of this measurement in medical practice and research.
Several problems arise in the evaluation and comparison of methods for the estimation of body fat percentage. For example, some of the nomenclature is ambiguous. The total body electromagnetic conductivity (TOBEC) method is distinct from the electrical impedance method, although they are often confused. All methods except direct carcass analysis depend upon the assumption of being robust in the face of inhomogeneity of location of the body components being measured. In the case of adipose tissue, this may be exemplified by the well known differences in fat distribution between men and women, during growth and development, and among ethnic populations. Perhaps less well recognized are the effects of differences in the general body conformation and varying distribution of blood, respiratory and enteric gas volumes. When used with inhomogeneous distributions of the body components being measured, the only truly robust body composition measurement methods are hydrostatic weighing and analogous methods.
Many methods, such as the electrical impedance and skin fold methods, require an indirect calibration through mathematical correlation with a criterion method or regression formula. Such a calibration can never be more precise than the hydrostatic weighing method. When they contain terms for weight and height and are sex, age and somatotype specific, regression formulas may actually contain little information about the method being evaluated. The test of statistical significance for a correlation coefficient (r) is surprisingly lenient, particularly for large experimental samples. With the number of samples (N) greater than 120, r need only be greater than 0.125 to achieve a probability (p) less than 0.05. Thus an investigator can report a "highly significant linear correlation" even when the data has little real value.
Hydrostatic Weighing is a direct extension of Archimedes' principle. The subject is weighed while immersed totally in water, and asked to exhale to remove as much of the air in his body as possible for a period of 90 seconds. The average density of adipose tissue (.about.0.9) and that of lean body mass (1.10) have been determined by various analytical means. Using these values, the analogous formula for percentage body fat (x) as a function of density (d) is: ##EQU1##
Because it is founded upon direct and well known physical principles without a need for indirect calibrations or limiting assumptions, hydrostatic weighing has become the "gold standard," or criterion method, for use in human subjects. However, one disadvantage of hydrostatic weighing is that it requires a very cooperative subject. Therefore, it is not suitable for babies or elderly, ill or physically challenged persons, or for comparative studies of other species. In addition, residual air in the lungs is a source of error.
Direct Carcass Analysis is the comparable criterion method for laboratory animals. In this ultimately invasive procedure, the subject is physically homogenized so that an aliquot sample can be subjected to extraction with an organic solvent. In addition to the death of the subject, the disadvantages of this method include the need for dangerous solvents and the availability of only one data point per subject.
Skin fold calipers and anthropometrics is the most commonly applied clinical method for men and women. This method uses measurements of skinfold thicknesses taken at the waist, chest and hip girth. This method is non-invasive, uses inexpensive apparatus, and is useful when performed by trained operators. However, anthropometric methods generally assume that the ratio of subcutaneous and other fat is constant within the group, and thus are limited to relatively homogeneous groups (e.g., young female gymnasts or young male weight lifters).
Electrical Impedance Measurement by current injection has received much recent interest, especially in the context of athletics and fitness programs. Electrical current at about 50 kHz is injected through electrodes on the subject's limbs to deduce electrical impedance. The subject is connected to a measuring device through four electrodes (usually two on an arm and two on the ipsilateral leg) and the electrical impedance is deduced from the voltage appearing on two passive electrodes when an electrical current at about 50 kHz is injected through the active electrodes. Although significant correlation with criterion methods has been claimed for this method, careful examination of these studies reveals these disturbing factors:
(1) The coefficients for the regression formulas for humans show that the contribution of the impedance to the calculation of fat percentage is actually rather small. Furthermore, the coefficient for the term(s) involving impedance varies widely among studies. The investigators recommend that age, sex and "fatness" specific formulas be used. In an isolated study where such additional sources of information were not used--a study in chickens where direct carcass analysis was the criterion method--the correlation coefficient was only 0.71, indicating that only half of the variation in body fat was "explained" by the impedance. PA1 (2) A recent study (Burkholder, W. (1991) Research Presentation) in dogs showed a very large (.about.40%) increase in body impedance during measurement under general anesthesia. This change was well correlated with rather moderate (.about.3 degrees F.) body temperature decrease and points to an important effect of changes in blood distribution in addition to vulnerability to inhomogeneity of fat deposition. The equipment is rather expensive and must be carefully designed to avoid danger from currents through the body. Additional limitations in nonhuman animals include hair, movement and extreme sensitivity to body conformation.
Total Body Electromagnetic Conductivity (TOBEC) is quite distinct from impedance measurement using current injection. With TOBEC, the subject is exposed to microwave energy in a chamber and the percentage body fat is assumed to be related to the differential absorption of microwave energy by fat and lean body mass. The raw TOBEC number is related to actual fat percentage using a regression formula. A controversial feature of this method is the large difference in TOBEC numbers as found in the same animals immediately before and after death. This change is directly related to changes in body temperature and is probably related to changes in blood distribution. Another disadvantage of this method is that the apparatus is expensive (more than $50,000 for human child size).
Total Body Water methods use administration of materials such as deuteriated or tritiated water, antipyrine, etc., which are assumed to be well and exclusively distributed in the "water space" of the body. Computations are based on assumptions about the relative distribution of water in the fat and lean body mass and assumptions about the distribution, fate and excretion of these materials. Correlation of these methods with hydrostatic weighing in adult healthy humans is good but correlation with criterion methods for animals is only marginal. This indicates that the computational model must be reformulated for each species and patient type. Some of the analytical methods are expensive and prolonged housing in metabolism cages is required for animals.
Total Body Potassium is a method based upon the relative distribution of potassium and its radioactive isotopes in the body water. The patient/subject is "counted" with a total body scintillation counter. The subject must be still for more than five minutes and body geometry factors are very important. In adult humans, the correlation with criterion methods is good. However, comparisons have not been reported in non-human animals. Furthermore, this method is tedious and the apparatus is very expensive.
U.S. Pat. No. 5,052,405, Oct. 1, 1991, titled "Method and Apparatus for Measuring the Density of an Object Including a Living Being," discloses a system based on Archimedes' principle for measuring the density of an object or living being. In this patent, the substance or subject is enclosed in a chamber and the temperature of the surrounding air is changed to vary its density. The apparent weight change of the subject at the different temperatures represents the change in the buoyant force exerted by the air on the subject. The buoyant force can then be calculated from the volume of the subject determined from its relationship with the buoyant force. The density of the subject can then be calculated from the volume and weight parameters and related directly to fat content. One disadvantage of this method is that it is constrained by the variation in temperature needed to produce a significant change in air density and buoyant force. Take, for example, a 70 kg subject with 25% body fat being measured at 55% relative humidity and 760 mmHg barometric pressure. A temperature change from 40.degree. C. (100.degree. F.) to 0.degree. C. (32.degree. F.) results in a weight differential of 10.96 g, i.e., only a 0.016% change. Therefore, this method is not useful for living subjects or any temperature sensitive substances.
International Patent Application PCT/US/W089/08428 discloses a method of measuring fat percentage using infrared absorption, which only sees the surface of a fatty region and offers very low accuracy.
U.S. Pat. No. 4,831,527 discloses a method in which tissue elasticity, pronounced in the stomach and buttocks, is used to produce variations in weight after an abrupt movement by a subject. The upward and downward forces caused by the movement of the stomach and buttocks are then correlated to overall fat percentage. This method is obviously highly dependent on fatty tissue location.
A number of patents (U.S. Pat. Nos. 4,449,406; 3,455,168; 3,557,625; and 3,487,698) describe methods for measurement of fat percentage in meat and are not suitable or adaptable for living subjects.
A series of patents have approached volumetric measurement of subjects as a means of arriving at body composition. U.S. Pat. No. 4,184,371 and U.S. Pat. No. 4,144,763 describe methods that do not account for the air volume in the lungs, resulting in significant inaccuracies. In U.S. Pat. No. 3,769,834, there is an attempt to deal with the respiratory error effects, but the method disclosed in the patent produces a large acoustic leak through tissue coupling from the chamber to the airways to the outside of the chamber. U.S. Pat. No. 4,369,652 describes a method that better accounts for lung volume but requires the subject to be wrapped in a down or polyester cocoon. In addition, this method requires elaborate precautions to control temperature.
In sum, the "gold standard" for measuring body composition in terms of fat percentage is underwater weighing based on measuring the density of the subject. Underwater weighing requires a fit cooperative subject as well as a tank of water adequate for total immersion. However, this method is clearly unsuited for small children, sick people or nonhuman animals. As discussed above, numerous other methods have been developed but these suffer from danger, discomfort, lack of precision, impracticality and great expense.